1. Field of the Invention
The present invention relates to a color image forming apparatus using electrophotography, and more particularly, to a technique to minimize a waiting time that occurs when a color mode is switched between a full color mode and a single color mode. The image forming apparatus refers to an apparatus for forming an image on a recording medium by using electrophotography. Specific examples of such image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (such as a laser beam printer, an LED printer, etc.), a facsimile machine, and a word processor.
2. Description of the Related Art
An image forming apparatus having an in-line second image carrying medium is known as one of color image forming apparatus using an electrophotographic process. As shown in FIG. 1, this type of image forming apparatus has image forming stations 10 (10Y, 10M, 10C, and 10Bk) for forming toner images of respective different plural colors. Each image forming station 10 (10Y, 10M, 10C, or 10Bk) has a photosensitive drum serving as a first image carrying medium 1 (1Y, 1M, 1C, or 1Bk) and a developing unit. The image forming stations 10 are arranged in line such that they face a second image carrying medium 7. The toner images of respective colors are transferred to the second image carrying medium 7 such that a multi-layer full-color toner image is formed on the second image carrying medium 7. The multi-layer full-color toner image formed on the second image carrying medium 7 is transferred to a recording medium 13 by a second-transferring device 8. This type of image forming apparatus has an advantage in that a high-quality image can be formed on a recording medium regardless of the type of the recording medium. Another advantage is that a full color image can be formed at a high speed. Because of these advantages, this type of image forming apparatus is widely used.
When a single color image is formed using the image forming apparatus having the in-line second image carrying medium, it is known to separate the photosensitive drums 1 (1Y, 1M, and 1C) of color image forming stations from the second image carrying medium 7 as shown in FIG. 2 such that the photosensitive drums 1 (1Y, 1M, and 1C) of the color image forming stations are not used when a single color image is formed. This method allows for disengagement of the photosensitive drums of the color image forming stations in the single color mode.
Japanese Patent Laid-Open No. 2004-004398 (corresponding to U.S. patent application publication No. 2003-0223785 A1) discloses a technique to reduce usage of photosensitive drums of color image forming stations during the process of forming a single color image. In this technique, after first transferring of all toner images is completed for a last page in a particular print job and before second-transferring is started, photosensitive drums (Y, M, C, and Bk) are separated from the second image carrying medium when second transferring for a page immediately previous to the last page is completed. This makes it possible to reduce the usage of the photosensitive drums without causing a color registration error when second transferring is performed. However, in this technique, it is needed to perform the separating operation when there is no toner image at the second-transferring position, and thus the time interval between the trailing end of one sheet and the leading end of a following sheet must be longer than the time needed to perform the separating operation.
Nowadays, it is very common to connect a printer or a copying machine having a printing function to a network so that a plurality of users can use the printer. In such an environment, various print requests are issued by various users at the same time, and thus it is needed to print full color images in the middle of a single color print job or vice versa.
It is needed to switch the color mode such that no color registration error occurs when the color image forming stations 10 (10Y, 10M, and 10C) are separated from or are brought into contact with the second image carrying medium 7. More specifically, when the color mode is switched from the full color mode to the single color mode, it is needed to perform the separation (or contacting) operation between the color forming stations 10 (10Y, 10M, and 10C) and the second image carrying medium 7 when a full color image formed on the image carrying medium 7 (or a single color image in the case where the color mode is switched from the single color mode to the full color mode) does not exist at the first-transferring position of the black image forming station 10Bk and does not exist at the second-transferring position.
In the conventional technique, to meet the above requirement, when the color mode is switched from the full color mode to the single color mode, the color image forming stations 10 (10Y, 10M, and 10C) are separated from (or brought into contact with) the second image carrying medium 7 after a full color image (or a single color image in the case in which the color mode is switched from the single color mode to the full color mode) formed on the second image carrying medium 7 has passed through the second-transfer position. Thereafter, forming of an image in the single color mode (or the full color mode) is started (FIG. 5). In this technique, however, if the color mode is changed frequently, a reduction in the throughput of the printing operation occurs.